The present invention relates to a transmission system for an aircraft, particularly a helicopter.
More specifically, although not exclusively, this transmission system is intended to transmit power (on a helicopter) between the main gearbox (associated with the main forward travel and lift rotor) and the rear gearbox (associated with the antitorque tail rotor) of the helicopter.
It is known that in order to achieve such transmission of power, a transmission system of a conventional type comprises a great many components such as tubes, connecting elements (flanges, flectors, etc.) and guide bearings. As a result, this conventional transmission system is particularly heavy, bulky, expensive and complex.
In addition, this high number of components means that a significant amount of lubrication needs to be provided, this of course giving rise to drawbacks (cost, risk of leakage, etc.). A known transmission system of this kind also entails having a number of bearings for the tubes, which bearings themselves also exhibit numerous drawbacks, such as the fact that they introduce stiffness, for example.
In order to at least partially overcome some of these drawbacks, a new type of transmission system known as a supercritical transmission system has been developed. A system such as this has far fewer mechanical parts and the transmission tubes have a greater diameter and are generally longer, which in particular makes it possible to reduce the number of bearings. In consequence, such a supercritical transmission system is lighter in weight, simpler to produce and less expensive.
However, this supercritical transmission system also differs from said conventional transmission system in that it has one or more natural frequencies (particularly the natural frequencies of bending of the transmission shaft) which are below the nominal rotational speed of the shaft (hence the name xe2x80x9csupercritical transmission systemxe2x80x9d), whereas for the aforementioned conventional transmission system (known as xe2x80x9csubcriticalxe2x80x9d), all the natural frequencies are above the nominal rotational speed.
In the context of the present invention, the nominal speed is to be understood as meaning the theoretical mean operating speed which is kept more or less constant for the flight of the aircraft.
In consequence, with such a supercritical transmission system, as the speed is increased, when the rotational speeds nears one of said natural frequencies, the vibration generated by this natural frequency is amplified and this may become particularly troublesome, particularly for the pilots and passengers of the aircraft and even, under certain circumstances, may become dangerous and in addition leads to accentuated wear of the parts of the transmission and other components of the aircraft.
The present invention relates to a transmission system for an aircraft, particularly a helicopter, that makes it possible to overcome these drawbacks.
To this end, according to the invention, said transmission system for an aircraft, of the supercritical type, comprising at least one rotary transmission tube which is such that at least one natural frequency of said transmission tube is below its nominal rotational speed, is notable in that it additionally comprises at least one active and controllable magnetic damper:
capable of generating a magnetic force capable of acting on said transmission tube; and
controlled in such a way as to generate a magnetic force that makes it possible to reduce the vibration of said transmission tube, at least when the speed of said transmission tube is within a predetermined range of frequencies defined about said natural frequency.
Thus, by virtue of the invention, as the speed increases (or decreases), when the rotational speed of the transmission tube nears said natural frequency, that is to say when it falls within the aforementioned range of frequencies, the magnetic damper according to the invention damps the vibration. In consequence, the usual amplification of the vibration as the natural frequency is passed through cannot take place, which prevents the occurrence of the aforementioned problems.
It will be noted that the damper according to the invention additionally makes it possible to replace one or more of the customary bearings of the transmission tube.
The transmission system according to the invention has numerous other advantages associated:
on the one hand, with the use of a magnetic damper rather than a customary bearing (for example a ball bearing); and
on the other hand, with the use of a supercritical system rather than a subcritical system.
Indeed, as indicated before, a supercritical transmission system is simpler, more lightweight, less bulky and less expensive than a conventional subcritical transmission system.
Furthermore, the use of at least one magnetic damper in particular affords the following advantages:
very low power consumption (approximately ten times less than for an equivalent ball bearing), a reduction in losses and an improvement in efficiency;
the absence of mechanical contact between the elements (hence an absence of wear), a longer life and greater reliability;
the possibility of obtaining very high transmission tube rotational speeds;
the possibility of operating under very arduous conditions, particularly at temperatures from xe2x88x92160xc2x0 C. to +250xc2x0 C. and in a very aggressive environment;
a regulating system (control loop) which is simple, as specified below;
the elimination of the oil and of the lubricating system;
the absence of contamination due to oil or grease; and
the possibility of dispensing with the conventional covers intended for transmission system maintenance.
Advantageously, said magnetic damper comprises:
a stator which comprises at least one electromagnet;
a rotor which is mounted on said transmission tube and which is situated in a zone of action of a magnetic force capable of being generated by said at least one electromagnet;
an electric current generator for producing an electric current capable of flowing through said electromagnet; and
means for controlling said electric current flowing through said electromagnet so as to control said magnetic force which acts on the rotor and therefore on the transmission tube.
As a preference, said magnetic damper comprises a number of electromagnets mounted in opposed pairs across said transmission tube.
In a preferred embodiment, said magnetic damper additionally comprises:
at least one sensor for measuring the lateral displacement of the transmission tube; and
a control loop which controls the generation of the magnetic force acting on the transmission tube as a function of the measurements taken by said sensor.
Thus, the magnetic damper is controlled automatically, simply and effectively, using this control loop. This avoids the intervention of an operator, particularly of the pilot.
However, such human intervention for controlling the operation of the magnetic damper may also be envisioned in the context of the present invention.
Moreover, advantageously, said magnetic damper is formed in such a way as to generate a magnetic force that makes it possible to shift at least said natural frequency of the transmission tube. As a preference, this shift is limited to roughly 3% of the initial value (prior to the shifting) of the natural frequency. To do this, the magnetic damper is designed in such a way that the residual stiffness it affords is below a predetermined value.
Furthermore, advantageously, the transmission system according to the invention additionally comprises:
at least one sensor for measuring the lateral displacement of the transmission tube; and
diagnostic means which, from the measurements taken by this sensor, deduce any malfunctioning there might be of the transmission system, and, in particular, of the magnetic damper.
Although not exclusively, the present invention applies more particularly to a helicopter equipped, in the customary way, with a main gearbox and with a rear gearbox.
In this case, advantageously, said at least one transmission tube of the transmission system is arranged in such a way as to connect said main and rear gearboxes.
As a preference, said transmission system comprises, in this case, a single transmission tube between the two gearboxes.
Thus it is conceivable for the customary bearings to be completely omitted.
Furthermore, advantageously, said magnetic damper is arranged between said main and rear gearboxes at a distance from said main gearbox which is roughly equal to one tenth of the total length of said transmission tube.